1. Field of the Invention
The present invention relates to a fuel cell. In particular, the invention relates to a fuel cell in which the dissolution of a catalyst or the like due to fuel starvation at an anode is suppressed.
2. Description of the Related Art
In this age, new technologies such as IT and biotechnology have evolved globally. Even in such circumstances, the energy industry remains as one of the largest basic industries in the world. In recent years, as environmental awareness including prevention of global warming has grown, the expectations regarding so-called new energy have increased. New energy has advantages in terms of environmental friendliness, minimization of power transmission loss and increased security of power supply as the energy can be produced in dispersed localized sites close to electrical power consumers. Furthermore, the development of new energy is expected to create new related industries as a spin off. Efforts for the development of new energy began in the early 1970s, triggered by the oil crisis. At present, the following types of energy are under development for practical use: reproducible energy produced by solar photovoltaic power generation and the like, recycled energy produced by waste power generation and the like, high efficiency energy produced by a fuel cell and the like, and energy use in new fields, typified by a clean energy car.
Among these, the energy produced by a fuel cell is one of the types of energy receiving the most attention from industries. A fuel cell generates electricity and heat simultaneously through the chemical reaction of atmospheric oxygen with hydrogen produced through the reaction of water vapor with natural gas, methanol, or the like and thus produces only water as a by-product of power generation. In addition to this, high power generation efficiency is obtained even in a low output power range, and electrical power generation is not affected by weather and thus is stable. In particular, a polymer electrolyte fuel cell has received attention as one of the next-generation standard power sources in applications such as vehicle-mounted use, mobile use, and stationary use in homes and the like. (See, for example, Japanese Patent Laid-Open Publication No. 2004-185830.)
As described above, residential power source system having a polymer electrolyte fuel cell, hydrogen (theoretically, a gas mixture of approximately 80% hydrogen and approximately 20% carbon dioxide) produced through the reaction of water vapor with natural gas or methanol is supplied to an anode. In a vehicle-mounted fuel cell system serving as a driving source for an automobile, pure hydrogen (almost 100%) stored in a hydrogen tank is supplied to an anode. Furthermore, in a power source system for mobile devices such as notebook personal computers and cellular phones, alcohol-based liquid fuel such as methanol is supplied to an anode. When such a fuel is sufficiently supplied to an anode and contributes to power generation, an anode reaction represented by equation (1) or (2) occurs, and protons (H+) move toward a cathode through an electrolyte layer.H2→2H++2e−  (1)CH3OH+H2O→6H++6e−+CO2  (2)
However, when a load is increased abruptly or when the concentration of the fuel decreases, so-called fuel starvation occurs in which the amount of fuel required at the anode during power generation becomes insufficient. In such a case, for example, carbon (C) in a catalyst-supporting carbon medium reacts with water (H2O) at the anode (and occasionally, also at the cathode) to produce protons (H+) as represented by equations (3) and (4), and thus a problem arises in that the carbon (C) is released as carbon dioxide (CO2). Furthermore, when the catalyst for the anode contains ruthenium (Ru), the reaction represented by equation (5) occurs, and thus a problem arises in that the ruthenium (Ru) is oxidized and dissolved as ruthenium oxide (RuO2). When the carbon (C) in the catalyst-supporting carbon medium is released, the supported catalyst (being Pt, Ru, or the like) is no longer supported, and thus the amount of the catalyst in the anode decreases. Furthermore, when the ruthenium (Ru) is oxidized and dissolved, a problem arises in that CO resistance is reduced.C+H2O→CO+2H++2e−  (3)C+2H2O→CO2+4H++4e−  (4)Ru+2H2O→RuO2+4H++4e−  (5)